1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for an engine.
2. Discussion of Background
In a conventional air-fuel ratio control apparatus disclosed in, for instance, Japanese Unexamined Patent Publication No. 204942/1983, an air-fuel ratio is detected by an air-fuel ratio sensor on the basis of the components of exhaust gas, and the air-fuel ratio is corrected according to an integrated value obtained by integrating the output of the air-fuel ratio sensor.
In the conventional air-fuel ratio control apparatus, however, the air-fuel ratio sensor could only determine two kinds of values: a rich side and a lean side. Accordingly, the method of controlling the air-fuel ratio by integrating the output of the air-fuel ratio sensor was permitted only to increase or decrease a fixed value per unit of time, and it was difficult to obtain sufficient control of an air-fuel ratio since a sufficiently converged value could not be obtained unless an output to be detected is present in an operational zone for a relatively long time when the correction coefficient is large, whereby it was difficult to obtain purifying operations for the exhaust gas. Further, it was necessary to provide a thick air-fuel ratio to increase the output of the engine when the engine is operated at a high revolution speed and a high load. Accordingly, information of correction on the air-fuel ratio could not be obtained in this operational region.